GMAT Prep software question - sequence DS

by **cdemisch** Sun Sep 23, 2007 11:39 pm

can't for the life of me figure this out. Can someone explain??
(numbers in parenthesis are subscripts)

If the terms of a sequence are t[1], t[2], t[3], ... t[n], what is the value of n?

(1) The sum of the n terms is 3,124.

(2) The average of the n terms is 4

According to the software the answer is C, both statements.[/i]

by **Harish Dorai** Tue Sep 25, 2007 10:01 am

Statement (1) :

t(1) + t(2) + .............. + t(n) = 3124.

Not sufficient as the sum doesn't give a clue for "n" (Which is the number of terms in the series).

Statement (2):

Average is 4. The average of this series is: t(1) + t(2) + ................. t(n) divided by n.

But the statement alone is not sufficient to find "n".

Combining the 2 statements, we know the Sum from statement (1) and we know the average from statement (2).

So Average = 3124/n = 4

So n = 3124 / 4 = 781.

So the answer is (C), as we can get n by combining the 2 statements.

by **RonPurewal** Mon Oct 01, 2007 5:35 am

Harish is correct again (and he's doing an admirable job of picking up our slack while we're behind!).

You should definitely have the following relationship at your fingertips: AVERAGE = TOTAL divided by # OF VALUES, or, equivalently, (AVERAGE) x (# OF VALUES) = TOTAL. I would place a large wager that you'll have to use it at least once on the test.